Technical side............................... Capaciter?

[TimeKeeper]

I am just curious about capacitor with different rating specially in microfarad.

I think, I understand when the capacitor is rated at 35V, that mean it will provide NO MORE THAN 35V of power.

How about microfarad? What's the different between 2200 vs 100? What does that mean?

Supposely I have device using 12V 35Watt, such as Auto HID headlight, then which capacitor should I use?

thanks,

 

[krotchy]

Farads = the amount of charge can be built on the capacitor based on the distance and SA of the plates. It is a true measure of the power a capacitor can provide.

The voltage rating is simply a limit for how much voltage you can put across the plates before it breaks down. Pretty much the higher the farad reading the longer a capacitor takes to charge/discharge, more it resists changes in voltage dv/dt. A large capacitor works very well as a high pass filter, while smaller ones work less effectively.

As far as stating 35V means no more than 35V of power, that is wrong, voltage is potential not power.

For example, a 1 Farad Cap with 30 volts across it is extremely dangerous and has a lot of stored charge/power, it could be used to generate a deadly current in theory.
A 1 nanofarad capacitor with 10,000 volts across it would be akin to getting zapped by a door knob on static-sensitive carpet.

As far as your question. The bigger cap (Bigger as in farads) is less likely to be effected by fluctuations in the power on your car. So for a 12V 35Watt headlight, I cant really tell you what the more useful cap is off hand as the smaller one might work just as well as the larger. But bigger is going to be no worse than the small one as far as preventing dimming due to your sound system.

 

[TimeKeeper]

Originally posted by: krotchy
Farads = the amount of charge can be built on the capacitor based on the distance and SA of the plates. It is a true measure of the power a capacitor can provide.

The voltage rating is simply a limit for how much voltage you can put across the plates before it breaks down. Pretty much the higher the farad reading the longer a capacitor takes to charge/discharge, more it resists changes in voltage dv/dt. A large capacitor works very well as a high pass filter, while smaller ones work less effectively.

As far as stating 35V means no more than 35V of power, that is wrong, voltage is potential not power.

For example, a 1 Farad Cap with 30 volts across it is extremely dangerous and has a lot of stored charge/power, it could be used to generate a deadly current in theory.
A 1 nanofarad capacitor with 10,000 volts across it would be akin to getting zapped by a door knob on static-sensitive carpet.

As far as your question. The bigger cap (Bigger as in farads) is less likely to be effected by fluctuations in the power on your car. So for a 12V 35Watt headlight, I cant really tell you what the more useful cap is off hand as the smaller one might work just as well as the larger. But bigger is going to be no worse than the small one as far as preventing dimming due to your sound system.

Argh, I got little more clear, more confused as well.
Let's say my HID headlight occasionally flickers. Dealer can't find anything wrong with it.
(let's assuming the ignitor and ballast is working properly)
I like to prevent "voltage shortage", what's the rough estimate of capacitor I should install? ( single 2200uf 35V adequate? )

thanks,

 

[Aluvus]

A capacitor is a device for storing charge.

The voltage rating is the maximum voltage (difference in potential) that the capacitor can safely see across its terminals.

The capacitance rating, in Farads, describes how much charge it will hold at a given voltage. A "large" capacitor has a higher capacitance rating. If you think of a capacitor as a bucket you store charge in, capacitance would describe how big the bucket is.

Because the Farad is a relatively large unit, practical capacitors are generally rated in units like microfarads (1 millionth of a Farad) or picofarads (1 trillionth of a Farad).

Caps are not generally useful for powering high-drain devices for extended periods of time (if they were, batteries would have faded from the market long ago). For instance, a capacitor can store energy as 1/2 * C * V^2. So a 1 F capacitor (which is pretty big) charged to 12 V would store 72 joules of energy. That would be enough to drive a 35 W device for just over 2 seconds.

In applications related to HID lighting, capacitors are more likely to be used to smooth out irregularities in the DC voltage supplied by the battery/alternator. Smoothing caps are all over the place. The exact spec you need will vary, consult the light manufacturer or someone well-versed in the subject.

 

[Rubycon]

Working voltage is the pressure of the electromotive force.

A capacitor is like a water tank. The Farad is quite a large amount of capacity hence µF or 0.000001 Farads. The working voltage is like psig on a tank. Exceed this and you risk bursting the tank. The capacitor's dielectric will break down when its working voltage is exceeded and it will fail suddenly causing it to heat up. With this heat comes an increase in internal pressure (with the case with electrolytic caps) and possible rupture or venting.

Folks will often put large capacitors close to amplifiers in mobile audio. This is to make up for voltage sags due to momentary peaks in power demand during loud musical passages. A capacitor in this scenario has an extremely low internal resistance and can dump its power very quickly. This prevents the voltage feeding the amplifier from sagging momentarily which can cause clipping and distortion long before the amplifier's maximum rated output is reached. Even if the battery is close to the amp it cannot provide this kind of peak current. Thus claims of adding a few Farads on the bass amp making the bass hit harder or tighter is justifiable. It will also stop annoying flickering of lights too. Of course if the power draw exceeds the capacity of the capacitor bank the voltage overall just drops. That happens when the electrical system is underpowered and capacitors are NOT designed for this.

 

[TimeKeeper]

thanks............ even though I still have no clue what I should get.

 

[krotchy]

Originally posted by: TimeKeeper
thanks............ even though I still have no clue what I should get.

as I said before. The calculations on this arent straight forward based on the information you have given. This is because the cap is being used purely as a decoupling/bypass cap, and there arent really any calculations for this without knowing alot more properties of your system than its a car and the load is 35W.

Pretty much bigger is better, so go with something reasonable in your budget and see if it works.

Here would be my best estimate, and this is based on very little if at all, so dont swear by it.
Using a simple calculation, we know the headlight has a resistance of roughly 4 Ohms (12^2/35 = R). So assuming a flicker is less than say 100ms, then to get an RC time constant less than this, you would need a 25mF or 25000uF cap to smooth out this signal.

Again I have no clue how long the flickering lasts for or if this is even valid, but that would be my best guess.

 

[TimeKeeper]

Originally posted by: krotchy

Originally posted by: TimeKeeper
thanks............ even though I still have no clue what I should get.

as I said before. The calculations on this arent straight forward based on the information you have given. This is because the cap is being used purely as a decoupling/bypass cap, and there arent really any calculations for this without knowing alot more properties of your system than its a car and the load is 35W.

Pretty much bigger is better, so go with something reasonable in your budget and see if it works.

Here would be my best estimate, and this is based on very little if at all, so dont swear by it.
Using a simple calculation, we know the headlight has a resistance of roughly 4 Ohms (12^2/35 = R). So assuming a flicker is less than say 100ms, then to get an RC time constant less than this, you would need a 25mF or 25000uF cap to smooth out this signal.

Again I have no clue how long the flickering lasts for or if this is even valid, but that would be my best guess.

The flicker happened once in the bluemoon but I am just kinda xxxxx want to make sure things always run smoothly.

NOW I am happy. Thanks~~~~~

 

[Bill Kunert]

Another thing about capacitors, there is no electron flow through them. They have decreasing resistance(capacitive reactance) as frequency increases. Just thought you ought to know.

 

[Aluvus]

Originally posted by: krotchy
Using a simple calculation, we know the headlight has a resistance of roughly 4 Ohms (12^2/35 = R). So assuming a flicker is less than say 100ms, then to get an RC time constant less than this, you would need a 25mF or 25000uF cap to smooth out this signal.

HID lights are not resistive elements. The input voltage is converted up to a higher voltage by an inverter.

The flicker is a result of the input voltage falling too low, and the device not being able to maintain a consistent flow of electrons.

 

[TimeKeeper]

Originally posted by: Aluvus

Originally posted by: krotchy
Using a simple calculation, we know the headlight has a resistance of roughly 4 Ohms (12^2/35 = R). So assuming a flicker is less than say 100ms, then to get an RC time constant less than this, you would need a 25mF or 25000uF cap to smooth out this signal.

HID lights are not resistive elements. The input voltage is converted up to a higher voltage by an inverter.

The flicker is a result of the input voltage falling too low, and the device not being able to maintain a consistent flow of electrons.

hmm.... little help here? thanks

 

[Rubycon]

Originally posted by: TimeKeeper

Originally posted by: Aluvus

HID lights are not resistive elements. The input voltage is converted up to a higher voltage by an inverter.

The flicker is a result of the input voltage falling too low, and the device not being able to maintain a consistent flow of electrons.

hmm.... little help here? thanks

Instead of a simple filament (tungsten-halogen) the HID fixtures use a driver that converts the 10.8-16VDC normally found in automotive electrical systems to the necessary voltage. (Often several kV to ignite the lamp and 30-90VDC at up to an amp to maintain the arc)

If the input voltage falls below the minimum on your lamp driver (or ballast) it can often cause the lamps to flicker or go out momentarily. Make sure all the connections are TIGHT.

 

[TimeKeeper]

Originally posted by: Rubycon

Originally posted by: TimeKeeper

Originally posted by: Aluvus

HID lights are not resistive elements. The input voltage is converted up to a higher voltage by an inverter.

The flicker is a result of the input voltage falling too low, and the device not being able to maintain a consistent flow of electrons.

hmm.... little help here? thanks

Instead of a simple filament (tungsten-halogen) the HID fixtures use a driver that converts the 10.8-16VDC normally found in automotive electrical systems to the necessary voltage. (Often several kV to ignite the lamp and 30-90VDC at up to an amp to maintain the arc)

If the input voltage falls below the minimum on your lamp driver (or ballast) it can often cause the lamps to flicker or go out momentarily. Make sure all the connections are TIGHT.

Thanks. I think I am getting there!!!!
Perhaps I need bigger capacitor w/ larger VDC?

 

[krotchy]

Originally posted by: TimeKeeper

Thanks. I think I am getting there!!!!
Perhaps I need bigger capacitor w/ larger VDC?

Not likely, since you are still trying to smooth the 10.8-16V input to the balast, not the output side of the balast.

 

[Rubycon]

A capacitor in this application is more of a band aid then a solution. Increasing WVDC gives a better margin against spikes (protection for the cap itself) but does nothing to increase buffer or hold time. The latter requires an increase in capacity, i.e. a higher mfd rating.

 

[TimeKeeper]

Originally posted by: Rubycon
A capacitor in this application is more of a band aid then a solution. Increasing WVDC gives a better margin against spikes (protection for the cap itself) but does nothing to increase buffer or hold time. The latter requires an increase in capacity, i.e. a higher mfd rating.

Here is what happen.
I would think OEM setting is adequate just to power up the HID. (everything in OEM, including stereo)

It flicker once in a bluemoon and I can't replicat that probem to the service technician.
I just want to make sure I don't want to see that flicker again. ( my warranty will run out this June, so I just want to make sure it will NOT happen AFTER that )

So......what's the alternatives?